This invention relates to locating subterranean formations of crude oil, and more particularly to determining porosity and permeability parameters of such formations.
Technological advances in crude oil exploration are permitting crude oil to be captured from locations previously considered to be impractical or unprofitable. For example, nuclear magnetic resonance (NMR) technology has been used for well logging applications to measure hydrogenous materials located a short distance into the earth""s structure about the bore hole. NMR can simultaneously sense the hydrogen in water and in oil and other materials that may be present within the sensitive measurement region, and thereby indicate the presence and amount of those materials.
Sometimes the water and oil constituents contributing to the total NMR response signal can be resolved to the allow concentration of each to be determined. In other cases, separate measurements of the oil and water may not be feasible with NMR alone.
When the hydrogenous material is contained within an underground rock formation, NMR techniques may be used to determine pore size distribution and the porosity and the permeability of the rock. With this information, a decision can be made whether a particular rock formation contains a sufficient amount of recoverable fluid such that drilling is profitable. However, NMR porosity and permeability estimates do not typically attempt to differentiate between the effect of varying proportions of oil and water in the fluid.
The invention uses a magnetic resonance technology, specifically electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR), to detect and measure the concentration of crude oil and certain other hydrocarbon solids and liquids contained within underground formations. Such detection and measurement may be obtained from the surface of the earth to appreciable depths below the surface. They may also be obtained from locations adjacent to the walls of natural openings in the earth""s surface (such as caves, open faults, cliffs, sink holes, and hillsides) or in man made earth penetrations (such as tunnels, wells, trenches or boreholes).
The use of EPR data is particularly advantageous in that EPR response signals emanate only from unpaired electrons, such as those due to broken bonds in high molecular weight (MW) hydrocarbon compounds, in paramagnetic and ferromagnetic materials, and in a few metals. In naturally occurring materials, broken bonds and paramagnetic ions are commonly found in, but not limited to, many crude oils, asphalts, and coals. The presence of these materials in the earth, or elsewhere, may be detected and measured by the invention.
The invention provides rapid detection and measurement as compared to other magnetic resonance methods, such as nuclear magnetic resonance (NMR). The time required to polarize and measure such electrons is commonly on the order of a few microseconds or less.
The invention also includes the use of EPR in combination with nuclear magnetic resonance (NMR) to provide additional advantages, particularly in well logging applications.